1. Field of the Invention
The present invention relates to a circuit for splitting poles between a first stage and a second stage of an electronic circuit, relying on the Miller effect.
It relates to the field of circuit design for components or electronic circuits, especially monolithic integrated circuits, in CMOS or other technology. It finds applications, in particular, in amplifier circuits.
2. Description of the Related Art
FIG. 1 is the diagram of an electronic circuit, for example an amplifier, comprising a first stage 10, a second stage 20 and a pole-splitting circuit 30 according to the prior art. The first stage behaves like a transconductance which delivers or draws a current through its output impedance. The second stage 20 is an inverting voltage amplifier. The output signal from this stage is therefore in phase opposition (180xc2x0) with respect to the signal at the input.
In this example, the second stage 20 is arranged in series with, and downstream of, the first stage 10. The pole-splitting circuit 30 is arranged between the output S1 of the first stage 10 and the output S2 of the second stage 20. It consists of the branch between the nodes S1 and S2 which comprises, in series, a capacitor Cc and a resistor Rz of relatively high value. This branch forms a feedback loop from the output S2 onto the output S1. The capacitance of this branch, which here corresponds to the capacitor Cc, is called loop capacitance. In this example, moreover, the first stage 10 is an operational amplifier and the second stage 20 is a power-output stage The latter consists of a power MOS transistor 21 in common-source mode, in series with a current source 22 between a power supply terminal delivering a positive power supply voltage Vcc, on the one hand, and the ground on the other hand. The transistor 21 here is an N-type MOS transistor, the output S2 of the stage 20 being taken on the drain of this transistor and the current source 22 being arranged between Vcc and this drain. Such a stage is therefore an inverting voltage amplifier.
The pole-splitting circuit 30 provides feedback and has the function of separating the respective poles of the first and of the second stage, so as to facilitate control of the stability of the feedback-type amplifier. More precisely, it makes it possible to shift, towards the low frequencies, the dominant pole p1 at the output S1 of the first stage 10. This is because, if the load resistance and the load capacitance at the output of the first stage 10 and at the output of the second stage 20 respectively are denoted Rout1 and Cout1 and Rout2 and Cout2 respectively, then:
the dominant pole p1 at the output S1 of the first stage 10 is given by:                     p1        =                              -            1                                Rout1            xc3x97            Cc            xc3x97            Av2                                              (        1        )            
xe2x80x83a second pole p2 is given by:                     p2        =                              -            1                                Rout1            xc3x97                          Cout2              Av2                                                          (        2        )            
xe2x80x83a third pole p3 is given by:                     p3        =                  1                      Rz            xc3x97            Cout2                                              (        3        )            
xe2x80x83and a zero z1 is given by:                     z1        =                  1                                    (                                                1                  gm2                                -                Rz                            )                        xc3x97            Cc                                              (        4        )            
xe2x80x83wherein, further, Av2 is the voltage gain of the second stage 20 and gm2 is the transconductance of the transistor 21 of the second stage 20.
According to the principle known as the Miller effect, the loop capacitance Cc is multiplied by the gain Av2 in expression (1) above. Stated otherwise, the capacitance Cc, which intervenes in the expression of the dominant pole p1 at the output S1 of the first stage, is seen, on this node, as being multiplied by the value Av2 of the gain of the second stage 20. This amounts to shifting the pole p1 towards the low frequencies.
In certain applications, the loop capacitance Cc has to withstand high potential differences. Thus, in the example represented in FIG. 1, in which the value of the signal Vout at the output S2 of the second stage 10 can vary between 0 V (volts) and Vcc, the voltage at the terminals of Cc can reach a maximum of Vcc-Vt, where Vt is the threshold voltage of a MOS transistor (typically 0.7 V). It results therefrom that the voltage at the terminals of Cc can exceed 10 V as soon as Vcc is higher than 10 V. However, the maximum value of the voltage which a capacitor, produced according to HF7CMOS technology, for example, can withstand is substantially equal to 10 V.
The pole-splitting circuit according to the prior art is therefore not suitable for this type of application.
Consequently, a circuit structure for splitting poles which is suitable in applications where the voltage at the terminals of the capacitor Cc can exceed the maximum voltage imposed by the technology used for fabrication would be desirable.
Aspects of the invention include a circuit for splitting poles between a first stage and a second inverting voltage-amplifier stage of an electronic circuit. The circuit comprises, on the one hand, in series between the output of the first stage and the output of the second stage, and in that order, a first capacitor, a second capacitor and a resistor, and, on the other hand, a voltage-divider bridge. The voltage-divider bridge is connected between a terminal delivering a substantially constant voltage and the output of the first stage. The output of the voltage-divider bridge is linked to the common node between the first capacitor and the second capacitor, in such a way that a first resistor of the voltage-divider bridge is connected in parallel with the first capacitor.
The fact of replacing the single loop capacitor Cc of FIG. 1 with two capacitors in series, and of imposing a defined potential at the common point between these capacitors by virtue of the voltage-divider bridge, makes it possible to reduce the maximum voltage which might be applied on each of these capacitors. The connection of the above voltage-divider bridge does not affect the feedback, even at low frequencies, whereby the Miller effect is maintained.
Another aspect of the invention relates to an electronic circuit comprising a first stage and a second stage the respective outputs of which are linked by a circuit for splitting poles as defined above.